There is a strong interest in flexible TFT back-panels because of the strong interest in wearable electronics. The key challenge in this endeavor is the substrate. To make devices with good electrical performance, the substrate has to survive high temperature processing steps and all kinds of chemical treatment without dimension deformations. Furthermore, the substrate has to remain flat for lithography purposes. No flexible substrate can achieve this requirement. The only solution is to mount the flexible substrate onto a flat and rigid substrate such as glass or ceramic (hereinafter generically referred to as ‘glass’) supporting sheet by adhesive for processing purposes and removal of the substrate by de-bonding after critical processing steps are completed.
There are a lot of challenges in finding the adhesive or glues and suitable flexible substrates that can maintain the dimension stability required. For most flexible substrates the coefficient of thermal expansion (CTE) is 50 to 100 ppm per degree Celsius. But for glass substrates the CTE is only a few ppm per degree Celsius. Therefore, during temperature rise of 300 degrees Celsius (common temperature required during normal processing), the plastic deformation can be as large as a few percent. Thus, there will be a strong stress to bend the glass substrate and it will be difficult for it to remain flat and to hold its shape. The thicker the plastic substrate, the stronger the force or stress is. For typical plastic substrates of 100 to 200 microns thick, the force is so strong that even the glass supporting sheet is bent. So it is advantageous to have the plastic substrate as thin as possible. But conversely, the plastic substrate has to be sufficiently thick to deal with handling after de-bonding.
The de-bonding process is also tedious and can require high temperature or laser illumination (depending upon the adhesive). The de-bonding process can damage the backplane and may not be compatible with display/sensor device processing, such as OLEDs or organic photodiodes. The OLED or organic photodiode cannot survive temperatures over 150 degrees Celsius. Even finished LCD cells cannot survive the high temperature that is normally required for de-bonding. Because of the potential damage to the electronic devices in the back-panel, the de-bonding process may be forced to be carried out before the display/sensor device processing is performed. If de-bonding has to be performed before display/sensor device processing, the display/sensor device processing has to be carried out on the flexible substrate, which is more difficult to accomplish. Therefore, it is very difficult to manufacture good flexible back-panels and flexible displays/sensors.
The advance of metal oxide TFTs (MOTFT) enables high performance TFTs to be fabricated at lower processing temperatures. However, decrease in performance and stability is still an issue even for MOTFTs made at low temperature. It is advantageous to be able to make flexible back-panels at high temperatures up to 300 degrees Celsius. Thus, the use of a rigid supporting element to hold a flexible substrate rigid during processing is still desirable.
In the prior art various attempts to fabricate flexible substrates on glass supports have been made. One example is described in U.S. Pat. No. 8,258,694, entitled “Method for Manufacturing Flexible Display Device Having an Insulating Overcoat and Flexible Display Device Having the Same”, issued Sep. 4, 2012 and a divisional thereof, U.S. Pat. No. 8,257,129. In this type of process, an insulating protection layer is formed on a rigid substrate (e.g. glass). Display elements are formed on the insulating protection layer and a flexible substrate is formed in an overlying relationship on the display elements. The rigid substrate is then removed by etching or the like. To perform the etching step, the material of the rigid substrate must have at least an etching selectivity 20 times greater than the insulating protection layer. It is important to note that the insulating protection layer does not stop the etching but is only etched at a much slower rate. That is to say, in all known prior art some of the insulating protection layer is removed in the process of etching the glass substrate. Since the amount removed affects the efficiency of the insulating protective layer (i.e. the protection provided by the layer) some compensation must be provided in advance. However, forming a thicker insulating protection layer in advance increases the stress on the glass substrate. Generally, many of these prior art fabrication methods have either been extremely difficult to use, usually because of the severe requirement for the selection of materials, or have failed completely because the etching step is too rigorous.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved process for fabricating an inverted flexible TFT back-panel on a glass support.
It is another object of the present invention to provide a new and improved process for fabricating an inverted flexible TFT back-panel on a glass support member wherein a step of removing the glass support member is performed prior to formation of display/sensor device processing.
It is another object of the present invention to provide a new and improved process for fabricating an inverted flexible TFT back-panel on a glass support member wherein steps of removing the glass support member and inverting the TFT back-panel are performed prior to formation of display/sensor device processing.
It is another object of the present invention to provide a new and improved process for fabricating an inverted flexible TFT back-panel on a glass support member wherein steps involving high temperatures or high resolution are performed in an appropriate order with the glass support member attached to reserve performance of the TFT array.
It is another object of the present invention to provide a new and improved process for fabricating flexible/conformable electronic array devices with processes requiring pixel level registration carried out with a dimension stable rigid supporting substrate and with other processes that do not need pixel level registration carried out after removing the rigid supporting substrate. It is another object of the present invention to provide a flexible/conformable TFT backpanel array devices for non-planar, wearable electronic devices, including displays, digital imagers, chemical and bio-sensors.